Start:
LET b0 = 0
IF PortB.1 = 1 THEN LET b0 = 1
SEROUT PortB.0, N2400, [b0]
PAUSE 10
GOTO Start

Don't forget the "ENDIF"

Start:
LET b0 = 0
IF PortB.1 = 1 THEN LET b0 = 1
* ENDIF*
SEROUT PortB.0, N2400, [b0]
PAUSE 10
GOTO Start